1. Field of the Invention
The present invention relates generally to the field of molecular biology. More particularly, it concerns methods and compositions involving microRNA (miRNAs) molecules. Methods and compositions for isolating, labeling, preparing miRNAs for analysis or as a tool for analysis are described, such as miRNA arrays. In addition, there are applications for miRNAs in diagnostics, therapeutics, and prognostics.
2. Description of the Related Art
In 2001, several groups used a novel cloning method to isolate and identify a large variety of “micro RNAs” (miRNAs) from C. elegans, Drosophila, and humans (Lagos-Quintana et al., 2001; Lau et al., 2001; Lee and Ambros, 2001). Several hundreds of miRNAs have been identified in plants and animals—including humans—which do not appear to have endogenous siRNAs. Thus, while similar to siRNAs, miRNAs are nonetheless distinct.
miRNAs thus far observed have been approximately 21-22 nucleotides in length and they arise from longer precursors, which are transcribed from non-protein-encoding genes. See review of Carrington et al. (2003). The precursors form structures that fold back on each other in self-complementary regions; they are then processed by the nuclease Dicer in animals or DCL1 in plants. miRNA molecules interrupt translation through imprecise base-pairing with their targets.
The function of most miRNAs is not known. A number of miRNAs, however, seem to be involved in gene regulation. Some of these miRNAs, including lin-4 and let-7, inhibit protein synthesis by binding to partially complementary 3′ untranslated regions (3′ UTRs) of target mRNAs. Others, including the Scarecrow miRNA found in plants, function like siRNA and bind to perfectly complementary mRNA sequences to destroy the target transcript (Grishok et al., 2001).
Some miRNAs, such as lin-4, let-7, mir-14, mir-23, and bantam, have been shown to play critical roles in cell differentiation and tissue development (Ambros, 2003; Xu et al., 2003). Others are believed to have similarly important roles because of their differential spatial and temporal expression patterns.
Research on microRNAs (miRNAs) is increasing as scientists are beginning to appreciate the broad role that these molecules play in the regulation of eukaryotic gene expression. The two best understood miRNAs, lin-4 and let-7, regulate developmental timing in C. elegans by regulating the translation of a family of key mRNAs (reviewed in Pasquinelli, 2002). Several hundred miRNAs have been identified in C. elegans, Drosophila, mouse, and humans. As would be expected for molecules that regulate gene expression, miRNA levels have been shown to vary between tissues and developmental states. Characterization of a number of miRNAs indicates that they influence a variety of processes, including early development (Reinhart, 2000), cell proliferation and cell death (Brennecke, 2003), and apoptosis and fat metabolism (Xu, 2003). In addition, one study shows a strong correlation between reduced expression of two miRNAs and chronic lymphocytic leukemia, providing a possible link between miRNAs and cancer (Calin, 2002). Although the field is still young, there is speculation that miRNAs could be as important as transcription factors in regulating gene expression in higher eukaryotes.
Several publications describe labeling miRNAs for analysis. These publications describe appending a radioactive phosphate at the 5′ end of the miRNA population using a polynucleotide kinase (Krichevsky, 2003) or a radiolabeled, single nucleotide at the 3′ end with RNA ligase (Dostie, 2003). For the purpose of using arrays to estimate the relative abundances of miRNAs in samples, these methods have two significant drawbacks: (1) only a single label is appended per miRNA, limiting the sensitivity that can be achieved and (2) the methods are compatible with radiolabeling only, which has disadvantages as compared to non-isotopic platforms for arrays. Furthermore, while RNA oligonucleotides have been labeled with non-isotopic labels (Martin et al., 1998), there is no evidence that small RNA molecules from a cell lysate can be effectively labeled in a similar manner after they have been enriched or isolated from the lysate.
Because microarrays are typically used to analyze messenger RNAs that are hundreds or thousands of nucleotides in length, we found that the 60-500mer probes typically used in microarrays were not compatible with miRNA analysis.
Therefore, there is a need for information about the function and activity of miRNAs, as well as for methods and compositions that can be used for their characterization and analysis.